1. Field of the Disclosure
The present disclosure relates to an optical device used for an optical communication system, and more particularly to a planar optical waveguide constituted by an optical waveguide including a clad layer and a core embedded in the clad layer which are formed on a substrate.
2. Discussion of the Background Art
A planar optical waveguide having an optical circuit such as an optical switch (for example, see Japanese Patent Publication No. 3755762 and Y. Hashizume, et. al., “Compact 32-channel 2×2 optical switch array based on PLC technology for OADM systems”, ECOC2003, M03-5-4) is used as an optical device for an optical communication system. ROADM (Reconfigurable Optical Add Drop Multiplexer) using a wavelength-multiplexed signal is one of optical communication system nodes constituted by the planar optical waveguides. This ROADM has a function of exchanging only a desired WDM channel signal in a node with a lower layer network and thereafter transmitting all the signals to an adjacent node, and this ROADM method is mainly used for constituting a ring network. In order to achieve this function, a high-functionality optical device in which planar optical waveguides are integrated in one module has been suggested.
FIG. 10 illustrates an example of a conventional planar optical waveguide. A light output from a MZI (Mach-Zehnder Interferometer) 51 constituting an optical circuit is divided into two, and one of the lights is input to a tap coupler 52. The light entered into the tap coupler 52 is divided into two, and one of the lights is output from an output end 53a as an optical output, and the other of the lights is output from an output end 53b as a monitoring output. A monitoring PD (Photo Detector) detects the intensity of the light output from the output end 53b. In the conventional planar optical waveguide, the MZI 51 and the tap coupler 52 are arranged substantially on the same straight line in order to increase the density of integration of the planar optical waveguide.
In the past, the tap coupler and the MZI are arranged substantially on the same straight line, therefore, a leaked light leaked from the core of the MZI to the clad layer enters into the tap coupler to vary a branching ratio.
Accordingly, it is an object of the present disclosure to prevent the leaked light leaked from the MZI from entering into the tap coupler and reduce the variation of the branching ratio.